Molarity and dilution
Molarity (M) is moles of solute per litre of solution. For dilution, the moles of solute stay the same, so M₁V₁ = M₂V₂. Add solvent and the concentration falls.
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Analogy
Squash cordial: adding water makes it weaker, but the amount of cordial is unchanged.
Example
Stock is 2.0 MM₁V₁ = M₂V₂2.0 × V₁ = 0.50 × 100 mLV₁ = 25 mL, add water to 100 mL
In practice
Labs keep a strong stock solution and dilute small amounts as needed, which saves space and gives accurate concentrations.
Practice
Flashcards
- Give the dilution equation.M₁V₁ = M₂V₂ (moles of solute stay the same).
Questions people ask
What is the difference between molarity and molality?
Molarity is moles of solute per litre of solution and changes slightly with temperature because volume changes. Molality is moles of solute per kilogram of solvent and does not change with temperature, because mass is fixed. Colligative property formulas use molality.
Why add acid to water, not water to acid?
Mixing strong acid with water releases a lot of heat. If you pour water into concentrated acid, the small amount of water can boil and spit acid out. Adding the acid slowly to a large amount of water spreads out the heat safely.